My interpretation: E-Mail - Messages sent much like regular mail. It can be sent to anybody as long as they have an email address. Messages reside on a server until they are picked up (mostly the messages still reside on the server until deleted). Instant Messaging - Restricted usually to the server that is providing the service. Mainly used for IMMEDIATE notifications of messages as soon as the individual is "online". Disadvantage - Individual has to be online in order to recieve an Instant Message (Hence - "Instant Message"). Programs such as ICQ and Yahoo Messenger allow sending of messages and notifying the user as soon as they are online (Kind of like an email client - Outlook, etc.)